One known rangefinding system for measuring a distance up to an object in a contactless manner employs a time-of-flight (TOF) process. According to such a TOF process, light that is emitted toward an object, and a period of time consumed after the light is emitted toward the object and until the light bounces off the object and returns, are measured, whereby the distance up to the object can be measured based on the period of time and the velocity of light (see Japanese Laid-Open Patent Publication No. 2001-281336, U.S. Pat. No. 5,754,280, and Ryohei Miyagawa and Takeo Kanade “CCD-Based Range-Finding Sensor,” IEEE Transactions on Electron Devices, Vol. 44, No. 10, October 1997, pp. 1648-1652 (hereinafter referred to as the “Miyagawa paper”).
The Miyagawa paper contains a detailed explanation concerning the timing at which pulsed light is emitted, as well as the timing of operations of a photosensitive device in a rangefinding system. More specifically, pulsed light is emitted and emission of the pulsed light is stopped repeatedly for identical periods (by a light-emitting device, which is energized at a duty ratio of 50%). The photosensitive device transfers photoelectrons alternately in two directions in synchronism with the emitted and non-emitted pulsed light (see FIG. 1 of the Miyagawa paper). A period of time consumed until the pulsed light bounces off the object and then returns is determined based on a difference between two output voltages of the photosensitive device.